If you get +35% power out of fitting one of them I will genuinely eat my own ****.
These sorts of products are generally best avoided, to be frank, along with the air temperature resistor mods and most other plug in boxes.
I would very much suspect they just fool the car in to thinking that it's colder than it is and bumps up the fuel rail pressure by fooling the sensor so it shoves a bit more fuel in.
If you have a diesel / jtd then you might get a small performance gain but that is likely to be accompanied by a lot of black smoke as she will run rich as hell, then later followed by some blocked cats I'd wager.
If you've got a petrol then you'll see even less of a gain.
Its one of those if it's worth doing it's worth doing properly jobs.
Turbo vehicles, in our case for Stilos therefore limited to turbo diesels, will remap well. A good map will be able to lift torque limiting factors in the lower gears, increase overall boost pressure across the rev range and adjust the fueling map to complement this. A turbo diesel will remap well, i'd expect a properly done jtd 115bhp to be pushing 140 bhp or more when mapped.
The petrol naturally aspirated vehicles you're only going to see gains of 5bhp probably; not very much at all particularly on the smaller displacement variants. Different supporting mods may yield a bit more than this of course. You may gain some drivability out of her thanks to throttle response and maybe better torque curves though, so it isn't all about power.